As is well known in the art, a conventional squirrel-cage motor has a large generator torque, which is the torque braking the motor when it rotates in the normal direction at an over-synchronous speed and with a negative slip. In the case of single-speed squirrel-cage motors this situation seldom occurs, but it occurs frequently in the case of pole-changing motors. In pole-changing motors, e.g. in the traversing and hoisting motors of cranes, a generator torque typically occurs when the braking effect of the generator torque is utilized in transitions from a higher motor speed to a lower speed. A large generator torque is a disadvantage because it has to be taken into account e.g. in the dimensioning of the gears. A large generator torque may also result in an undesirably high deceleration rate.
An excessive generator torque can be reduced by adjusting the stator voltage by means of various regulators during the time when a generator torque is present.
The motor structure itself can also be used to influence the magnitude of the generator torque and the rotational speed at which the maximum generator torque occurs. One of the previously known methods used to reduce generator torque is to increase the number of turns in the stator or rotor windings.
Similarly, the sectional form of the stator or rotor grooves can be utilized to influence the magnitude of the generator torque. In the case of small motors, however, a sufficient groove depth cannot be achieved because of the small size of the motor.
As is known in the art, the inclination of the grooves can also be used as a means of influencing the magnitude of the generator torque and the rotational speed at which a maximum generator torque occurs. A change in the groove inclination also effects the magnitude of slip on the motor side of the motor's power curve. A large motor slip is a drawback in parallel-operated drive motors, particularly in traversing or hoisting mechanisms of cranes, because it results in skewing of the crane or its load.
Groove inclination in this context means the inclination of the grooves in terms of a percentage of the pole pitch. In conventional asynchronous motors the groove inclination is, at its maximum, about 40% of the pole pitch.
In motors provided with cast windings, groove inclinations exceeding this value have not been used for the purpose of altering the generator torque. The normal groove inclination, i.e. an inclination of about 40%, has been used in motors with cast windings only for reducing the noise levels and overharmonic torques of the motor, and for improving the motor performance.
The circumstances referred to above constitute a limitation on the use of pole-changing motors with cast windings.